Females do not have an extra rib. According to the Encyclopedia Britannica, both men and women normally have 24 ribs. Christian website Answers in Genesis explains that the Biblical story of Eve's creation is the source of the common misconception.
According to the Biblical story found in Genesis, God first created Adam, then Eve. According to Answers in Genesis, God created Eve from one of Adam's ribs. As a result, some have thought that all men have one fewer rib than women. Even if the story of Eve's creation is taken to be literally true, it does not indicate that women would always have an extra rib; it only means that Adam had, after that moment, one less rib than Eve.